(I raised this in GMF; this is the resulting bug report.)

An attempt to merge into a topic with the title or subject line "Challenge treadle sewing machine , can't open top of table" gives a corrupted message in the prompt to confirm the merge, and if confirmed, does not merge. The exact text of the subject line seems both necessary and sufficient to provoke the problem: the title and contents of the topic to be merged, and the contents of the topic into which to merge, do not seem to matter.

The confirmation message reads, Are you sure you wish to merge the topic 'Thread being merged from' into the topic 'Action'? The word Action is not the name of the topic. The merge itself, if confirmed, does not work. The original topic is in my group vintagesewingmachinerepair.

 - Mark; with thanks to Bruce for help.